This invention relates generally to electrical contacts on SiC-based semiconductor devices, and more particularly to ohmic and rectifying contacts on silicon carbide semiconductors.
SiC possesses tremendous advantages for high temperature and high power solid state electronics. In addition, it offers potential advantages for high frequency and logic circuit applications: e.g., power conversion (mixer diodes, MESFETs), single chip computers (n-MOS, CMOS, bipolar transistors), non-volatile random access memory SiC CCDs can hold charge for more than a thousand years thus, for example, making hard disks a thing of the past.
The potential maximum average power, maximum operating temperature, thermal stability, and the reliability of SiC electronics far exceeds Si or GaAs based electronics. The degree to which these advantages of SiC can be utilized, however, is presently constrained by the thermal stability and electrical properties of the metal/SiC junctions. The primary reasons for this are: (1) the power density of SiC devices is limited by the thermal stability of the ohmic contact junctions, and (2) substantial cooling is required to insure the stability of electrical contact junctions.
For a long time, researchers have been striving without success to develop electrical contacts to silicon carbide trying overcome these constraints. Until these constraints are removed, SiC devices/circuits offer only marginal--if any--advantages over Si and GaAs. Utilization of the full performance potential of SiC itself (for all devices), requires four types of performance-limiting electrical contacts: (1) ohmic to p-type SiC, (2) ohmic to n-type SiC, (3) rectifying to p-type SiC, and (4) rectifying to n-type SiC.
The value of SiC electronics lies in its potential to extend the capabilities of solid state electronics beyond what is possible with Si or GaAs. Thus, suitable electrical contact characteristics obtained in the laboratory--under low stress conditions--must not drift or degrade, due to changes at the metal/SiC junctions, under actual device operating conditions. This requires two additional attributes of metal/SiC electrical contacts. First, the contact metal must form a junction with SiC that is chemically stable to approximately 1000.degree. C. (joule heating at high forward current densities in power SiC devices could easily cause metal/SiC junctions to reach such temperatures). Second, the contact metal (or metallization structure) must act as a diffusion barrier to circuit and bonding metals (electrode metals) at the same temperatures. Metal/SiC electrical contacts demonstrated previously do not come close to meeting all these stability requirements.
The best known p-type SiC ohmic contact metallizations demonstrated to date are Al, Al/Ti and Pt (Refs. 2,3). Although specific contact resistances as low as 1.times.10.sup.-5 .OMEGA..multidot.cm.sup.2 have been reported in laboratory studies, actual SiC devices exhibit p-type contact resistances of 1.times.10.sup.-2 .OMEGA..multidot.cm.sup.2 to 1.times.10.sup.-3 .OMEGA..multidot.cm.sup.2 (Refs 10, 11).
Aluminum and its suicides, and Al/Ti form thermally unstable interfaces with SiC and/or have melting problems (Ref. 12, pg. 2). Platinum reacts with SiC to form many different silicides at temperatures as low as 280.degree. C. At temperatures as low as 400.degree. C., Pt continues to react with SiC until it is entirely consumed by the formation of PtSi (Refs. 4, 5). Thus, Pt cannot protect its contact from circuit and bonding metal diffusion; this characteristic of Pt, in contact with SiC, requires that Pt itself not be used as a circuit or bonding metal. Further, PtSi reacts with virtually all suitable circuit and bonding metallizations at very low temperatures.
Virtually every electronic device--unipolar or bipolar--requires n-type ohmic contacts. The most successful n-type SiC ohmic contact metallizations demonstrated to date are Ni and TiC. Both exhibit specific contact resistances between 1.times.10.sup.-5 .OMEGA..multidot.cm.sup.2 and 1.times.10.sup.-6 .OMEGA..multidot.cm.sup.2 (Ni: Ref. 13; TiC: Refs. 7&9). Nickel forms silicides at very low temperatures. Its graded Ni-silicide junction is thermally unstable, and it cannot form a protective diffusion barrier to circuit and bonding metals. Other Ohmic contacts formed of silicide, nitride, carbide or multiple layers of such materials exhibit stability problems similar to Ni contacts. Transition metals such as Mo and W do not adhere well to SiC surfaces, and they spall at elevated temperatures.
TiC forms an electrical ohmic contact junction with n-type SiC, that is stable to at least 1400.degree. C. (Ref. 14). However, it reacts with all candidate circuit and bonding metals at low temperatures; thus, it cannot form a protective diffusion barrier to circuit and bonding metals.
Low work function metals and semi-metals, that do not react with SiC to form tunnel junctions, can be used to form rectifying electrical contacts to p-type SiC. This type of electrical contact has not been required for SiC devices demonstrated to date; thus, there is little, if any, background literature. We have demonstrated that TiC forms an excellent rectifying contact to p-type SiC, that is stable to at least 1400.degree. C. (Ref 14.). However, it reacts with all candidate circuit and bonding metals at low temperatures; thus, it cannot form a protective diffusion barrier to circuit and bonding metals.
The n-type, rectifying Schottky diode is required to modulate the current and voltage in all majority carrier solid state devices. The most important devices of this type are Schottky diodes and MESFETs. Excellent adhesion and abrupt metal-SiC interfaces are cornerstone requirements for this type of contact. Metallizations previously developed for this purpose exhibit various deficiencies. For example, attempts to make thermally stable Schottky rectifying junctions on n-type SiC have been stymied. Many of the metals that are capable of forming such junctions (e.g., Ti, Au/Ti, Pt and PtSi) react with SiC to form silicides at relatively low temperatures, or--in the case of Au--do not adhere to the SiC surface. The electrical contact junction is thermally unstable, leading to compositional grading of the junction interface at elevated temperatures. This grading effect is exacerbated by the creation of free carbon at the original metal/SiC interface, causing substantial performance degradation, and limiting SiC device operation to well below the capabilities of SiC itself. The (W/Ti, Ti, Au/Ti and Pt)/SiC n-type Schottky junctions demonstrated by Cree Research, N.C. State, NASA Lewis and the Japanese, are unstable at high temperatures or under sustained high current density conditions.
The best rectifying contact metal to n-type SiC, demonstrated to date appears to be W/Ti, extensively tested at the U.S. Army Res. Labs in Adelphi, Md. This contact, however, exhibited unstable leakage-current characteristics, probably due to thermally-driven reactions at the interface. The testing consisted of cycling the junctions between room temperature and 500.degree. C. (number of cycles unknown), and measuring the junction reverse leakage current I(L) at room temperature as a function of the number of cycles. During a first series of cycles, I(L) increased. During the next series of cycles, I(L) decreased, reaching a minimum value about 10 .mu.A greater than it was before stress testing was begun. Thereafter, I(L) did not change.
U.S. Pat. No. 2,918,396 to Hall describes formation of rectifying and ohmic contacts on silicon carbide PN diodes and PNP and NPN transistors by alloying two silicon-acceptor or silicon-donor alloy globules to an N-type or P-type silicon carbide crystal at high temperature, (1700.degree. C.). The alloying metals disclosed include aluminum, phosphorus, tungsten, molybdenum or tungsten-molybdenum, with nickel or tungsten conducting electrodes.
U.S. Pat. No. 3,308,356 to Rutz discloses a method of making rectifying contacts to PN junctions in a silicon carbide substrate bonded on one side to a tungsten block by alloying fragments of silicon doped with Ga (P-type) or As (N-type) onto the exposed face of the SiC substrate in a forming gas atmosphere (90% N, 10% H) at high temperature.
U.S. Pat. No. 3,510,733 to Addamiano discloses forming on SiC semiconductor devices electrical leads of an alloy consisting primarily of chromium and nickel, but which can include traces of Si, C and Fe.
U.S. Pat. No. 4,738,937 to Parsons describes a method of making nonrectifying (ohmic) Schottky contacts on a semiconductor substrate, such as Si or SiC, by epitaxially depositing a metal having suitable work function and lattice parameters, the identified metals including Yb on Si, NiSi.sub.2 and W on N-type Si, and TiC on N-type .beta.SiC.
U.S. Pat. No. 5,442,200 to Tischler discloses forming an ohmic contact on a SiC surface by depositing a sacrificial silicon layer followed by a metal (Ni, Cr, Pd, Ti, W, Ta, Mb, Co, Zr or mixtures or alloys thereof to form a noncarbonaceous ohmic contact structure.
U.S. Pat. No. 5,448,081 to Malhi proposes a MOSFET device formed on a SiC substrate, with doped source and drain regions but does not disclose any means for connecting electrodes to such regions.
U.S. Pat. No. 5,471,072 to Papanicolaou discloses a Pt rectifying contact and a Ti/Au ohmic contact on n-type SiC. Both forms of contacts degrade at high operating temperatures, e.g., subject to catastrophic degradation at over 800.degree. C. as disclosed and probably unstable at temperatures of 500.degree. C. or less.
Although the semiconductor materials .alpha. and .beta.-SiC have a demonstrated capability for stable, efficient performance at high temperatures, the same is not true of SiC devices/circuits, due to the instabilities of their electrical contact structures.
Also, applicant has learned that a TiC contact on .beta.SiC does not, by itself, form a diffusion barrier to circuit or bonding metals. Appropriate circuit/bonding metals such as W, Pt, Au and Pd, form intermetallics with TiC. These solid state reactions change the composition of the electrical contact junction, thus degrading it.
Accordingly, a need remains for thermally stable ohmic and rectifying electrical contacts to n-type and p-type SiC.